Did you change the oil yourself, or did you take it in to have it done?

If you took it in, there is a very real possibility that you are now low on transmission fluid. The 4L30E transmission is a sealed unit -there is no dipstick to check the fluid level. On the bottom of the pan, there are two plugs -the lower for drainage, and the higher for fill/check.
It is possible that the pit jockey either pulled the wrong plug to check the level, or they let fluid out when checking the level from the top plug (easy to do when the engine is running).

The transmission is a decent unit, it just requires above average attention to detail on the part of the service department. It is not Isuzu specific, as it's a GM model, and found in many vehicles.
I'd recommend that you take it in for a transmission service -somewhere that knows for certain how to check and fill a sealed transmission.